Natural gas pipeline operators typically send instrumented platforms called ‘pigs’ through the pipelines in order to monitor the structural integrity thereof. The cost for such procedures can be as much as $1 M for each 100 miles of inspected pipe. The position and velocity of the pig at all times must be known so that pipe observations may be accurately correlated with location information. Presently, odometers are utilized for this purpose; however, these devices are inaccurate because of slipping, and recalibration against markers in the pipe such as weld joints at approximately every 30 m must be made. For differences greater than 30 m, the odometer has insufficient accuracy. Another approach is to use a sonde device where a low-frequency electromagnetic wave (˜500 Hz) penetrates the pipe and can be detected a few meters above the ground. Observers present at designated locations receive the sonde signal as the pig passes nearby through the pipe. The location is then marked using GPS, but the accuracy of the measurement is a few meters. Moreover, for the method to work the pipe cannot be buried too deep in the ground. Because of these inaccuracies, if a defect is located, a significant length of pipe must be uncovered in order to locate the defect identified by the pig. This may be time-consuming, expensive and inconvenient especially if the pipe is located beneath a busy city thoroughfare or under a river, as examples.
Microwave radiation having, for example, a frequency of 10.5 GHz (X-Band) has a smaller wavelength (3 cm) than the diameter of a typical gas pipeline; that is, greater than 6 cm in diameter, is known to propagate inside metal pipes without significant attenuation, the pipe behaving as a waveguide for such radiation. Higher microwave frequencies may be employed for pipes having smaller diameters, but higher attenuation at these higher frequencies may reduce the range of transmission at these wavelengths. Microwave radiation has also been observed to readily travel through bends in pipes.
In “A Low-Cost Dynamic Range-Finding Device Base on Amplitude-Modulated Continuous Ultrasonic Wave” by Hong Hua et al., IEEE Trans. Instr. and Meas. 51, 362 (2002), a discussion is provided for the ultrasonic determination of distance longer than the wavelength of a high frequency carrier wave using a low frequency amplitude modulated signal.